The present application relates to photoconductors and, in particular, to photoconductive semiconductors for detecting light, i.e., photodetectors or for switching voltages, i.e. photoconductive switches. Such devices are used in a variety of applications ranging from the study of the physics of superconducting thin films [W. J. Gallagher, et al., Appl. Phys. Lett. 50, 350 (1987)] to use in the electro-optic sampling technique [J. A. Valdmanis et al., in "Picosecond Optoelectronic Devices", edited by C. H. Lee (Academic Press, NY, 1984), p. 249], for probing integrated circuits and measuring the microwave scattering parameters of high-speed semiconductor devices. Other potential applications include use as a detector for optical communications, optical interconnects for VLSl circuitry and packaging [G. Turner et al., paper presented at the MRS Spring Meeting, Reno, NV (1988)], and as an element for optical signal processing [A. G. Foyt et al., in "Picosecond Optoelectronic Devices", edited by C. H. Lee (Academic Press, NY, 1984, p. 271]. In the switching applications, it is of primary importance that the device be capable of high speed operation, i.e., that the rate of conductivity change is high enough to respond to very fast changes in photoexcitation intensity.
Typically, a photoconductive device consists of a semi-insulating semiconductor body upon which an electrically conductive strip is formed. A small gap is made in the strip. A voltage is applied to one side of the strip. When the gap is illuminated with an intense light pulse that is absorbed near the semiconductor surface, a photogenerated electron-hole plasma is formed creating a conductive path across the gap, allowing current to flow. In this state, the switch is considered to be switched ON. When the light pulse is turned off, the current in the photoconductive detector, or switch, turns off with a decay time characteristic of the recombination lifetime of the semi-insulating semiconductor. When no light is incident, little current flows from one conductor to the other through the semi-insulating material.
In response to fast light pulses, most standard semi-insulating semiconducting films exhibit a long turn-off transient compared to the speed of the light pulse due to long recombination lifetimes of the semiconductor material.